One thing is to be familiar with the series you 
collect. Often times, you are able to pinpoint 
the exact moment of the exact episode with a cel--
 by comparing it to the episode itself, you can 
see whether or not it's a perfect match. This 
isn't always a foolproof method-- occasionally, 
you have sequences that are animated that end up 
being cut or edited before filming-- but those 
instances will be the exceptions and not the 
rules.

However, you're probably not going to have to 
worry too much about fake cels, unless you happen 
to collect a very expensive series. For the most 
part, if you buy from reputable, established cel 
dealers, you will be purchasing a genuine 
product. If you buy cels from ebay or Yahoo!
Japan, you run into problems. For example, a 
description can be worded to be technically 
accurate, yet misleading. Or if you don't know 
Japanese at all, it might state clearly that the 
item isn't what you think it is... but Babelfish 
might mangle the translation so much the caveat 
is lost on you.

If you get into collecting cels, you're going to 
want to educate yourself on what you're buying. 
Often times, it's just a mixture of common sense, 
familiarity with the series, and knowledge of 
what other collectors with similar tastes already 
possess. Occasionally, the situation may come up 
where you might want second, third, and fourth 
opinions on a situation, and that's where good 
friends and public forums come in handy.

But the best thing is to buy your first pieces of 
animation art from a reputable source, to become 
familiar with them... and then branch out with 
time.